1. Field of the Invention
This invention relates to a thermoelectric material for generating electrical energy by the use of temperature differences (based on the Seebeck effect) or for generating temperature differences from electrical energy (based on the Peltier effect). More particularly, it relates to a thermoelectric material consisting of an oxide with a perovskite structure, wherein the oxide is composed of a rare earth element, an alkali earth metal element, and a transition metal element. The thermoelectric material of this invention exhibits larger thermoelectric power at low temperatures below room temperature, as compared with conventional thermoelectric materials.
2. Description of the Prior Art
As is well known in the art, the thermoelectrical property, which is expressed by the efficiency of conversion between thermal energy and electrical energy, can be evaluated in terms of a thermoelectric figure of merit Z. The value of Z can be estimated by the equation Z=.alpha..sup.2 .lambda./k. Here, .alpha. is the thermoelectric power (.mu..multidot.V.multidot.K.sup.-1), .lambda. is the electrical conductivity (.OMEGA..sup.-1 cm.sup.-1), and k is the thermal conductivity (W.multidot.cm.sup.-1 K.sup.-1). As can be seen from the equation, a larger .alpha., a larger .lambda., and a smaller k are necessary to obtain a larger thermoelectric figure of merit Z.
However, it is well known that .lambda./k for metals is expected to be a constant by the Wiedemann-Franz law, and that the three constants mentioned above are not necessarily independent of each other. This law is not always applicable to the case of semiconductor materials, and the selection of materials to produce a thermoelectric material is therefore relatively unrestricted. Moreover, the value of .alpha. obtained for metals is in the order of 10 .mu..multidot.V/K, whereas a number of semiconductors have a higher value of .alpha. than that obtained for metals by one or more orders of magnitude.
Examples of typical thermoelectric materials that have been previously developed include polycrystalline silicides of transition metals used for thermoelectrical power generation, and chalcogenides of bismuth or antimony used for cooling based on the Peltier effect. Among them, Bi.sub.2 Te.sub.3 -type compounds have the most excellent thermoelectric figures of merit at temperatures around room temperature, and it is well known in the art that they have come into use as Peltier elements in electronic cooling apparatuses and the like.
However, this material exhibits a drastic decrease in the thermoelectric figure of merit at temperatures below room temperature, so that the cooling temperature is restricted at present only within an extremely narrow temperature range around room temperature. For this reason, it has been required to develop a thermoelectric material for cooling based on the Peltier effect, which has a large thermoelectric figure of merit over a wide range of temperatures below room temperature.